Article surveillance systems that work on the principle of detecting the presence of a particular form of ferromagnetic material in a periodically changing low level magnetic field are well known. U.S. Pat. No. 4,553,136 illustrates a representative system of this type in which an article to be protected against unauthorized removal from a defined area, such as a store or library, has a marker attached to it that has in it a strip of low coercivity ferromagnetic material of high permeability. When the article, with the marker attached, is brought into an interrogation zone in which the low level periodically changing magnetic field is applied via a radiating antenna located at the edge of the zone, the presence of the strip modifies the pattern of the applied field, generating magnetic sub-fields at harmonics of the fundamental frequency of the applied field. These harmonic fields are picked up by a receiving antenna also located at the edge of the interrogation zone and fed to a receiver where they are detected and used to trigger an alarm to signal the unauthorized removal of the article. Other representative examples of ferromagnetic marker systems of this general type are found in U.S. Pat. Nos. 3,665,449; 3,747,086; 3,820,104 and 4,710,752.
To allow the article to be removed from the area without triggering an alarm, the marker may be physically removed from the article by an authorized person using a special tool. Alternatively, as disclosed in U.S. Pat. Nos. 3,665,449 and 3,820,104, for example, a high coercivity ferromagnetic strip placed adjacent the low coercivity strip in the marker may be changed from an unmagnetized to a magnetized state at the time of sale or checkout to render the marker ineffective in the low level field of the interrogation zone.
When magnetic markers are employed with magnetic media, such as cartridges containing audio or video tapes, care must be taken to make certain that the surveillance system does not compromise the information recorded on the magnetic media. U.S. Pat. Nos. 4,499,444 and 4,665,387, for example, show surveillance systems for magnetic tape products using a magnetic marker placed in the cartridge but physically separate from the magnetic tape media. Carefully designed apparatus disclosed in each of the patents is employed to deactivate the marker without compromising the recorded information. This is done by magnetizing an adjacent high coercivity strip in the marker using a highly concentrated (localized) magnetic field that does not extend to any harmful degree to the magnetic tapes and thus does not deleteriously affect the magnetic tape media itself.
There is a need for surveillance systems to provide security protection for computer disk magnetic media. With the proliferation in business and government of personal computers and the ready portability and easy concealibility of the miniature data disks used by these computers it is important to provide systems comparable to the above described types that ensure that sensitive data is not lost or compromised intentionally or unintentionally by unauthorized removal from secure areas in which the data is normally used. While non-magnetic marker systems are available for surveillance purposes and would have the advantage of not interfering with the magnetic media of data disks, they are generally not physically suited to the miniature size and slim geometries of conventional magnetic, optical and magneto-optical data disks employed with personal computers. Use of the ultra thin magnetic strips are highly desirable for this purpose.
One proposal for a surveillance system for magnetic data disks involves the attachment of a low coercivity, high magnetic permeability ferromagnetic marker strip to an inside surface of the jacket in which the disk is housed. This, however, would allow the system to be easily defeated by removing the disk from the jacket and reinserting it into another, non- protected jacket with the data on the disk still intact. Moreover, data on a protected disk cartridge could be transferred by a simple computer copy process onto an unprotected disk cartridge and safely removed from the area.
Unlike the case of clothing and books in which the surveillance system protects the physical article, computer data disks present the more intricate problem of the need to protect the data recorded on the disk from unauthorized removal even when the article itself, i.e. the disk, is not removed.
It is therefore an object of the present invention to provide article surveillance apparatus and system especially adapted for use in computer data disk, particularly of the mini and micro disk size.
It is a further object of the present invention to provide article surveillance apparatus and system that protects the security of the recorded data irrespective of concern for the physical condition of the article on which the data is recorded.
It is a still further object of the invention to provide a system for ensuring that data to be protected is not compromised by copying onto unprotected media.